Math Homework Assignment

A student, working on a rather long math homework assignment,
discovered that one problem was fairly easy to solve, except that it
required about three pages of fairly simple proof after the one or two
difficult steps. It being rather late at night, he did the difficult
steps and left the proof undone, along with a note:

“This proof is left as an exercise for the grader.”

Next week, he received his homework back. He noted that several extra
pages had been stapled to the back of it. Examining the extra pages,
he was surprised to find the entire proof written down step-by step.
At the end, in red pen, the grader had written: